Prior attempts to provide a high degree of fire retardancy to ordinarily flammable unsaturated polyester resins have involved the addition of one or more of brominated or chlorinated compounds, phosphates, antimony compounds or other additives to the unsaturated polyester resin, or the preparation of unsaturated polyesters containing bromine or chlorine in the polymeric structures. None of these approaches has proved to be completely satisfactory. For example, bromine or chlorine containing compounds incorporated in the resin generate poisonous gas upon combustion, and phosphates cause degradation of unsaturated polyester resins by hydrolysis.
Basic magnesium carbonate has been widely employed in the resins as a temperature sealing material in order to enhance fire retardancy. However because of its high degree of oil absorbency, only a small amount of basic magnesium carbonate (up to about 30% by weight) can be incorporated in the resin.